Visible light emitting device formed from wide band gap semiconductor doped with a rare earth element

ABSTRACT

A visible light emitting device includes a wide band gap semiconductor layer doped with one or more elements which emit light at various wavelengths based upon atomic transitions. The semiconductor preferably is GaN, InN, AIN, BN or alloys thereof doped with a lanthanide element such as Er, Pr or Tm. The light emission can be enhanced by annealing the WBGS.

BACKGROUND OF THE INVENTION

Light emitting diodes (LED) and related light emitting devices are usedin a vast number of applications. These can be used in most lightemitting devices from simple panel lights to complex displays andlasers. Currently LEDs are used in the automotive industry, consumerinstrumentation electronics, and many military applications. Differentcompounds are used to produce different wavelengths of light. Forexample, aluminum gallium arsenide is used for red LEDs, galliumaluminum phosphide for green, and GaN for blue. Light emitting materialsformed from three different materials are often difficult to produce.Utilizing different LEDs together inherently requires allowing fordifferent performance characteristics such as current and voltagerequirements.

Wide band gap semiconductors (WBGS) doped with light emitting elementssuch as rare earth elements (RE) and other elements with partiallyfilled inner shells are particularly attractive for LEDs because theemission efficiency appears to increase with band gap value, thusallowing room temperature operation without the need to introduceimpurities. Wide band gap generally refers to a band gap of 2 eV orgreater. Electroluminescence has been reported from several WBGS hostsincluding Er-doped gallium arsenide, gallium phosphide, GaN, ZnSe andSiC. Er-doped semiconductor light emitting diodes have been shown toemit in the infrared at about 1.5 microns. The infrared emissioncorresponds to transmissions between the lowest excited state(⁴I_({fraction (13/2)})) and the ground state (⁴I_({fraction (15/2)}))of the erbium atoms. The first Er-doped semiconductor light emittingdiodes emitted IR light only at very low temperatures. However, recentadvancements have permitted IR light emission at near room temperature.Although IR emitting Er-doped GaN has a great deal of utility in thecommunications industry, it previously has not been useful in a lightemitting diode requiring visible emission.

SUMMARY OF THE INVENTION

The present invention is premised on the realization that wide band gapsemiconductor substrates doped with elements with partially filled innershells such as rare earth elements and transition metals can be formedand will emit in the visible and ultraviolet spectrum at a wide range oftemperatures. The wide band gap semiconductor material are group III-Vand IV materials including diamond, GaN, AIN, InN, BN and alloysthereof. These are doped with elements such as cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, turbium,dysprosium, holmium, erbium, thulium, ytterbium, or lutetium or otherelements with partially filled inner shells.

By proper formation of the wide band gap semiconductor material andproper introduction of the rare earth element, a light emitting diodecan be formed which emits in the visible spectrum.

By selection of the appropriate dopant material, one can select theappropriate color. For example, in GaN, erbium will produce greenwhereas thulium will produce blue and praseodymium will produce red.

The objects and advantages of the present invention will be furtherappreciated in the light of the following detailed description anddrawing in which:

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a graph depicting the PL spectrum of Pr-implanted GaNfilms treated under different annealing conditions.

DETAILED DESCRIPTION

In order to form a light emitting devices according to the presentinvention, a wide band gap semiconductor material is formed on asubstrate and doped with an effective amount of a rare earth element.The substrate itself can be any commonly used substrate such as silicon,silica, sapphire, metals, ceramics and insulators.

The WBGS is either a group III-V material or a group IV material such asdiamond. In particular the WBGS material can include III-Vsemiconductors such as GaN, InN, AIN, BN as well as alloys of these. Anyproduction method which forms crystalline semiconductors can be used.Suitable techniques include molecular beam epitaxy (MBE), metal-organicchemical vapor deposition (MOCVD), chemical vapor deposition (CVD),plasma-enhanced chemical vapor deposition (PCVD), hydride vapor phaseepitaxy (HVPE) and PECD. The desired thickness of a WBGS material willbe formed on the substrate. For emission purposes the thickness of theWBGS is not critical. For practical reasons the thickness of the WBGSlayer will be from about 0.2 to about 5 microns, with around 1 to 2microns being preferred.

For the rare earth or transition metal to be strongly optically activein the wide band gap semiconductor, group III deficient growthconditioners should be utilized. This should permit the rare earthelement to sit in an optically active site which promotes the higherenergy or visible light emission.

The dopant material is one which has a partially filled inner shell withtransition levels that can result in visible or U.V. emission. Thedopant material can be a transition metal such as chromium or a rareearth element preferably from the lanthanide series and can be any ofcerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, turbium, dysprosium, holmium, erbium, thulium, ytterbium, orlutetium. Typically RE dopants include thulium for a blue display,praseodymium for a red display, and erbium for a green display. Thesecan be added to the WBGS by either in situ methods or by ionimplantation. Generally the concentration can be relatively high, fromless than about 0.1% up to about 10 atomic percent. The dopantconcentration can be increased until the emission stops. Generally, thepreferred concentration will be about 0.1 to about 10 atomic percent.

Further a full-color display can be created by utilizing threeoverlapping WBGS layers such as GaN each layer doped with differentlight emitting rare earth elements. Separate wiring could be used foreach layer and each layer could be separated by transparent insulatinglayers. An array of side by side light emitting diodes could also beused to provide a full color display. A combination of dopants in thesame WBGS can also be employed.

It may be desirable to anneal the WBGS. This tends to increase emissionup to a point. Generally the WBGS is annealed in an argon or other inertenvironment at a temperature 800-1200° C. for 1-5 hours. More preferablythe temperature will be from 850-1050° C., most preferably about 950° C.

The invention will be further appreciated in light of the followingdetailed example.

EXAMPLE 1

An erbium-doped GaN Schottky contact LED emitting visible light wasformed by growing an erbium-doped GaN film in a Riber MBE-32 system on atwo inch pSi substrate. Solid sources are employed to supply the gallium(7 N purity) and erbium (3 N) fluxes while an SVTA rf plasma source isused to generate atomic nitrogen. In this application, a GaN bufferlayer was first deposited for 10 minutes at a temperature of 600° C.followed by GaN growth at a temperature of 750° C. The growth conditionswere as follows: nitrogen flow rate 1.5 sccm at a plasma power of 400Watts, gallium cell temperature of 922° C. (corresponding to a beampressure of 8.2×10⁻⁷ torr) and erbium cell temperature of 1100° C. Theresulting GaN growth rate was about 0.8 microns/hour, and the erbiumconcentration was about 10²⁰/cm³. GaN films with a thickness of 2.5microns were utilized.

To fabricate Schottky diodes on the GaN:erbium films, a semitransparentaluminum layer was deposited by sputtering. The aluminum film waspatterned into a series of ring structures of varying areas utilizing alift-off process. The aluminum rings serve as individual Schottkycontacts while a large continuous aluminum surface was used as a commonground electrode. Electro luminescence characterization at ultravioletand visible wavelengths was performed with a 0.3 m Acton researchspectrometer fitted with a photo multiplier tube detector. Allmeasurements were conducted at room temperature using dc applied biasvoltage and current.

Applying reverse bias current to the order of 1 milliamp to a GaN:erbiumSchottky LED, results in green emission visible with the naked eye undernormal ambient lighting conditions. The emission spectrum consists oftwo strong and narrow lines at 537 and 558 nm which provides the greenemission color. The two green lines have been identified as erbiumtransmissions from a ²H_({fraction (11/2)}) and ⁴S_({fraction (3/2)})levels to the ⁴I_({fraction (15/12)}) ground state. Photo luminescencecharacterization of the same GaN erbium films grown on silicon performedwith a helium cadmium laser excitation source at a wavelength of 325nanometers, corresponding to an energy greater than a GaN band gap, alsoproduce green emissions from the same two transitions. Minor EL peakswere observed at 413 and at 666/672 nanometers.

The device had a threshold voltage for forward conduction of about 8.5volts. At a forward voltage of 20 volts, a current flow of 350 milliampsis obtained. Under reverse bias of 20 volts, a current of about 30microamps is measured. The capacitance voltage characteristic of thediode has a voltage intercept of about 11.5 volts and an effective GaNcarrier concentration of approximately 10¹²/cm³. The high diode forwardresistence obtained in the current voltage characteristics of about 34kilo-Ohms is probably due to the high resistivity of the GaN layer. TheSchottky barrier height calculated from the capacitance voltagecharacteristics is approximately 9 volts, which was consistent with thethreshold voltage. This large voltage probably indicates the presence ofan insulating layer on the aluminum-GaN interface.

A linear relationship is maintained between the optical output and thebias current over a wide range of values. At current values smaller than200 milliamps, the relationship is linear.

EXAMPLE 2

Er-doped GaN films are formed in a Riber MBE-32 system on c-axissapphire substrates. Solid sources were employed to supply the Ga (7 Npurity), Al (6 N), and Er (3 N) fluxes, while an SVTA Corp. rf plasmasource was used to generate atomic nitrogen. The substrate was initiallynitrided at 750° C. for 30 min at 400 W rf power with a N₂ flow rate of1.5 sccm, corresponding to a chamber pressure of mid-10⁻⁵ Torr. An AINbuffer layer was grown at 550° C. for 10 minutes with an AI beampressure of 2.3×10⁻⁸ Torr (cell temperature of 970° C.). Growth of theEr-doped GaN proceeded at 750° C. for 3 hours with a constant Ga beampressure of 8.2×10⁻⁷ Torr (cell temperature of 922° C.). The Er celltemperature was varied from 950 to 1100° C. The resulting GaN filmthickness was nominally 2.4 μm giving a growth rate of 0.8 μm/h, asmeasured by scanning electron microscopy (SEM) and transmission opticalspectroscopy. Photoluminescence (PL) characterization was performed withtwo excitation sources: (a) above the GaN band gap-HeCd laser at 325 nm(4-8 mW on the sample); (b) below the GaN band gap-Ar laser at 488 nm(25-30 mW). The PL signal was analyzed by a 0.3 m Acton Researchspectrometer outfitted with a photomultiplier for ultraviolet(UV)visible wavelengths (350-600 nm) and an InGaAs detector for infrared(1.5 μm) measurements. The PL signal of the Er-doped GaN samples wasobtained over the 88-400 K temperature range. Above band gap excitation(He—Cd laser) resulted in light green emission form the Er-doped GaNfilms, visible with the naked eye.

Two major emission multiplets are observed in the green wavelengthregion with the strongest lines at 537 and 558 nm. A broad emissionregion is also present, peaking in the light blue at 480 nm. The yellowband typically observed at ˜540-550 nm in GaN PL is absent.

EXAMPLE 3

Pr-doped GaN films were grown in a Riber MBE-32 system on 2″ inch (50mm) p-Si (111) substrates. Solid sources were employed to supply the Gaand Pr fluxes, while an SVTA rf-plasma source was used to generateatomic nitrogen. The growth of GaN:Pr followed the procedure previouslydiscerned for GaN:Er. Substrate growth temperature was kept constant at750° C. and the Pr cell temperature was 1200° C. We estimate, based onour work with GaN:Er, that this cell temperature results in a Prconcentration in the range of 10¹⁸-10²⁰/cm³. PL characterization wasperformed with He—Cd and Ar laser excitation sources at wavelengths of325 and 488 nm, respectively. The PL and EL signals were characterizedwith a 0.3-m Acton Research spectrometer outfitted with aphotomultiplier tube (PMT) detector for UV-visible wavelengths and anInGaAs detector for IR. To measure EL characteristics, contacts wereformed by sputtering a transparent and conducting indium-tin-oxide (ITO)layer onto the GaN:Pr structure.

He—Cd PL excitation (as 325 nm) resulted in an intense, deep redemission from the Pr-doped GaN, visible with the naked eye. The roomtemperature PL at visible wavelengths is shown in FIG. 1 for a 1.5 μmthick GaN film grown on Si. The spectrum indicates a very strongemission line in the red region at 650 nm, with a weak secondary peak at668 nm.

EXAMPLE 4

Praseodymium implantation was performed in a MicroBeam 150 FIB systemutilizing a Pr—Pt liquid alloy ion source (LAIS). The Pr—Pt alloy wasprepared by mixing praseodymium and platinum at an atomic percent ratioof 87:13. This produces an eutectic alloy with a melting point of 718°C. Mass spectrum analysis showed that a Pr²⁺ target current of ˜200 pAwas produced, representing 75% of the total target current. A Pt⁺ targetcurrent of ˜50 pA was also observed.

The Pr²⁺ beam was accelerated to high voltage and implantation wascarried out at room temperature on GaN films grown by MBE, HVPE, andmetalorganic chemical vapor deposition (MOCVD). After FIB implantation,the samples were annealed under different conditions. PL measurementswere performed at room temperature by pumping the samples with a CWHe—Cd laser at 325 nm. The He—Cd laser was focused on the samplesurface, where the laser power and beam diameter were 12 mW and 200 μm,respectively. The PL signal was collected by a lock-in amplifier andcharacterized with a 0.3-m Acton Research spectrometer outfitted with aphotomultiplier tube (PMT) detector for UV-visible wavelengths and anInGaAs detector cooled to 0° C for IR. A grating of 1200 grooves/mm witha resolution of 1.67 nm/mm was used for UV-visible wavelengths.

The FIGURE shows the annealing effect on PL intensity for a Pr-implantedGaN film grown on sapphire by MBE. The implanted pattern is a 136 μm×136μm square. The implantation was performed using a 300 keV Pr²⁺ beam witha target current of 200 pA. The pixel exposure time was 1.14 ms and thepixel size was 0.265 μm×0.265 μm. This results in a dose of ˜1×10¹⁵atoms/cm². Simulation of these implantation conditions using TRIM'95⁹calculates a projected range of ˜60 nm and a peak concentration of˜1.7×10²⁰ atoms/cm³. The sample was first annealed at 950° C. for onehour in flowing argon. After this first anneal, the 650 nm peak becamediscernible. The sample was subsequently annealed at 950° C. for anothertwo hours, leading to an increase in the peak intensity at 650 nm. Thethird anneal was carried out at 1050° C. for one hour resulting in thePL intensity at 650 nm increasing by a factor of 4. In spite of thesmall implanted pattern size (136 μm×136 μm), the emitted red lightintensity was strong enough to be easily seen with the naked eye.Annealing for a fourth and final time at 1050° C. resulted in a reducedPL intensity. This suggests that a one-step annealing at 1050° C. isadequate to optically activate the Pr³⁺ ions implanted in the GaN film.Similar PL spectra were observed from Pr-doped sulfide glasses.

EXAMPLE 5

A GaN region was also patterned by Pr FIB implantation. The implantationwas performed using a 290 keV Pr²⁺ beam for a dose of ˜4.7×10¹⁴atoms/cm². After FIB implantation, the sample was annealed at 1050° C.for one hour in Ar. Under UV excitation from the He—Cd laser, theimplanted region emits red light, while unimplanted surrounding areashows the yellow band emission of GaN.

EXAMPLE 6

Pr implantation was also performed on GaN films grown by HVPE and MOCVD.Regions consisting of 141 μm×141 μm squares were implanted on bothsamples with a dose of 1×10¹⁵ atoms/cm² and a beam energy of 290 keV.Both samples as well as a Pr-implanted MBE sample (dose=4.7×10¹⁴atoms/cm²) show strong red emission at 650 nm, which corresponds to the³P₀ _(^(→)) ³F₂ transition of Pr³⁺. All three samples show similar bandedge emission at around 365 nm.

EXAMPLE 7

Pr-implanted GaN film grown by MBE on sapphire was formed. After FIBimplantation with a dose of 4.7×10¹⁴ Pr/cm² the sample was annealed at1050° C. for one hour in Ar. The Pr concentration of the in-situ dopedGaN film is estimated to be at the range of 10¹⁸-10²⁰ atoms/cm³. Ingeneral, the PL intensity of the in-situ Pr-doped GaN sample is stronger(˜5×) than that in the FIB-implanted sample, which is expected from themuch larger Pr-doped volume which is excited in the former case. For thesamples, the full width at half maximum (FWHM) of the 648 and 650 nmlines are ˜1.2 nm, which corresponds to 3.6 meV.

Thus the present invention can be utilized to produce light emittingdevices from wide band gap semiconductor material utilizing rare earthdopants. The particular wavelength of emission is certainlycharacteristic of the added component. Further, it is possible tocombine the rare earth implants to develop unique light emittingdevices. Thus the present invention lends itself to a wide variety ofdifferent light emitting devices, extending from the infrared range downthrough the ultraviolet range.

This has been a description of the present invention along with a preferred method of practicing the invention. However, the invention itself should be defined only by the appended claim, wherein we claim:
 1. A light emitting device which emits light in the visible to UV spectrum comprising a semiconductor material selected from one of group III-V and group IV wide band gap semiconductors and having embedded in there a light emitting element in an amount effective to provide emission in the visible to UV spectrum, said element comprising a light emitting element having a partially filled inner transition level suitable for light emission in the visible spectrum or higher wherein said device is adapted to receive a voltage effective to cause said light emitting element to radiate visible light.
 2. The device claimed in claim 1 wherein said wide band gap semiconducting material selected from the group consisting of galium nitride, aluminum nitride, indium nitride, boron nitride, alloys thereof and diamond.
 3. The light emitting device claimed in claim 2 wherein said light emitting element is selected from the group consisting of transition metals and rare earth elements.
 4. The device claimed in claim 3 wherein said light emitting element is selected from the group consisting of rare earth elements.
 5. The light emitting device claimed in claim 4 wherein said rare earth element is selected from the group consisting of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
 6. The light emitting device claimed in claim 5 wherein said rare earth element is selected from the group consisting of thulium, praseodymium, and erbium.
 7. The light emitting device claimed in claim 4 having at least two separate rare earth elements.
 8. The light emitting device claimed in claim 1 wherein said semiconductor includes from about 0.1 to about 10 atomic percent rare earth element.
 9. The light emitting device claimed in claim 8 wherein said semiconductor material is formed by molecular beam epitaxy.
 10. The light emitting device claimed in claim 1 wherein said semiconductor material is GaN.
 11. The light emitting device claimed in claim 1 wherein said light emitting element is chromium.
 12. The device claimed in claim 1 wherein said device is adapted to receive at least about 8.5 volts.
 13. A light emitting device comprising three overlapping semiconductor layers selected from the group consisting of AIN, BN, GaN, InN and alloys thereof wherein each of said layers includes an amount of a separate rare earth element effective to provide light emission of each of said layers at different wavelengths said light emitting device adapted to receive a voltage effective to cause each of said rare earth elements to radiate visible light.
 14. The device claimed in claim 12 wherein said device is a schottky diode. 